py-celery: updated to 4.3.0

4.3.0:
- Added support for broadcasting using a regular expression pattern
  or a glob pattern to multiple Pidboxes.
- Added support for PEP 420 namespace packages.
- Added :setting:acks_on_failure_or_timeout as a setting instead of
  a task only option.
- Added the :signal:task_received signal.
- Fixed a crash of our CLI that occurred for everyone using Python < 3.6.
- Fixed a crash that occurred when using the Redis result backend
  while the :setting:result_expires is set to None.
- Added support the DNS seedlist connection format for the MongoDB result backend.
- Bump the minimum eventlet version to 0.24.1.
- Replace the msgpack-python package with msgpack.
- Allow scheduling error handlers which are not registered tasks in the current
  worker.
- Additional fixes and enhancements to the SSL support of
  the Redis broker and result backend.

(adam)